1. Field of the Invention
The invention relates to a mixing and recirculation cycling system in the stable section of a paper or cardboard machine and to a method for the mixing and recirculation of stock suspensions, backwater, and return flows in a paper or cardboard machine, and particularly in the stable section.
2. Description of Background Information
A mixing and recirculation cycling system for stick suspensions is known from U.S. Pat. No. 4,477,313 which issued to Anderson on Oct. 16, 1984. According to this patent, the backwater collected in the paper machine is passed into open backwater tanks and, after being provided with thick stock in mixing pumps, fed again to the headbox.
A similar device and a similar method are known from German published Application No. DE 195 09 522 A1. In this invention, a headbox sectioned over the machine width is fed with a stock suspension through a large number of stock suspension feed lines from a distributor. Part of the backwater appearing in the drainage section of the paper machine is sectionally returned to the headbox and is used for weight basis control according to the dilution principle.
In paper and cardboard machines with known embodiments of the stable section, degrees of temporal basis weight fluctuation and paper composition fluctuation appear that are no longer acceptable, due to increasing machine speeds as well as simultaneously rising quality specifications for paper.
In order to show the operating environment of the invention, a customary related art mixing and recirculation cycling system in a stable section of paper and cardboard machines is depicted by way of example in FIG. 1. However, the invention also operates with differently implemented cycling systems, and differently designed cycling systems do not depart from the framework of the invention.
FIG. 1 is a schematic illustration of a wet section 24 of a paper or cardboard machinery with a dilution water-regulated headbox (i.e., the headbox is regulated by dilution water). Backwater is taken from the wet section and fed via a backwater inlet 3 into a backwater tank 1. Headbox recirculation 4 from a headbox 23, deculator overflow 22 from a deculator 29, and accepted stock from a second vertical separator stage 26 also flow into the backwater tank 1. First and second cleaner stages 27 and 28 feed the deculator 29, which in turn feeds the backwater tank 1 and a first vertical separator stage 25. A first portion of the backwater is passed from the backwater tank 1 to the headbox 23 via a separation device, which is a vertical separator 30 in this case. Most of the backwater mixes with the fresh stock. The excess backwater from the cycle of the stable section may escape from the cycling system via a backwater discharge line 31.
FIG. 2 depicts a known backwater tank 1, into which the backwater inlet 3 is introduced. Simultaneously, the headbox recirculation 4, the accepted stock from the second vertical separator stage 5, the accepted stock from the second cleaner stage, and possibly the deculator overflow 6 are introduced into the upper region of the backwater tank 1. Fresh stock is fed into the lower region of the backwater tank via line 8.
Discharge lines 9 for the dilution water for the vertical separator and the cleaner system, then the backwater inlet 10 to the headbox for dilution water regulation and, in the lowest region and the stock suspension inlet 11 to the first cleaner stage or to the deculator, are disposed from top to the bottom of the discharging lines from the backwater tank, as seen in FIG. 2.
In this type of design, extreme stock density fluctuations in the backwater tank occur due to temporally unstable currents of the backflows. For example, the headbox recirculation of the accepted stock of the vertical separator stage, or of the accepted stock of the second cleaner stage has these problematic fluctuations. The blending of the return flows in the usually large backwater tank also results in the disadvantage that, at the time of a change in the type of paper, it takes a long time before temporally stable relationships in volume flow and stock density are reestablished.
Another known design of the mixing and recirculation cycling system in the stable section of a paper machine is depicted in FIG. 3. The backwater feed 3 flows into the open top of the tank, and a mixing tube 2 is introduced in the bottom region of the tank. The headbox recirculation 4, the accepted stock of the second vertical separator stage 5, and the accepted stock of the second cleaner stage 6 and possibly also the deculator overflow run into the mixing tube 2. A feed line 8 for the addition of fresh stock, which is also introduced in the lower region of the backwater tank, is concentrically provided in the mixing tube 2. The addition of fresh stock may be adjusted by a control valve 12 and a flowmeter 13. Through the concentric arrangement of the fresh stock feed 8 in the mixing tube 2, thorough mixing of the fresh stock with the remaining suspension added and with the backwater is achieved. At the upper region of the backwater tank 1, the backwater inlet 10 is connected to the headbox, and at the bottom region near the fresh stock feed, the stock suspension is pumped from the backwater tank via the stock suspension inlet 1 to the first cleaner stage or to the deculator with the help of a pump 12.
While this design of stock piping avoids the introduction of the return flows directly into the backwater tank, a significant disadvantage of this design is that the flow rate at the outlet of the mixing tube 2 is too low for thorough mixing, since the difference in fluid levels .DELTA.h.sub.geo of the mixing tube and the tank 1 is excessively low. Thus, only slight flow rate differences can be generated in the region of the mixing point.